Apparatus for strapping packages with weldable plastic bands, in particular apparatus in which a closure unit is provided as a separate unit, are already known from a number of publications. The closure methods used in such cases are usually welding methods such as friction welding or welding by means of a welding tongue. Thus, for example, CH-A-686 079 discloses an automatic pack-tying apparatus of the abovementioned type having a closure unit, with a first and a second clamping device arranged directly beneath a slide plate which can be moved back and forth. The slide plate absorbs forces when the plastic band is clamped firmly during the closure operation, but moves away, following completion of the closure operation, in order to release the package. The clamping devices are parts which can be moved up and down and have toothed or otherwise suitably shaped top parts for firmly clamping the plastic band between the movable parts and the slide plate. Clamping devices of similar construction are also disclosed in reference publications DE-A-44 25 908, DE-A-40 14 307 and EP-0 490 477.
In the case of all these apparatus, the clamping action of the clamping device is achieved by translatory movement of a clamping part. Apparatus of this type have the disadvantage that, with high tensioning, there is a risk of the plastic band slipping in the firmly clamped state. U.S. Pat. No. 011,807 and EP-A-0 099 606 disclose apparatus with pivotably mounted clamping parts with curved operative surfaces. However, with these apparatus there is also a risk, with high tensioning, of the plastic band slipping in the firmly clamped state.